Low back pain and injuries attributed to manual lifting activities continue as one of the leading occupational health and safety issues facing preventive medicine. Despite efforts at control, including programs directed at both workers and jobs, work-related back injuries still account for a significant proportion of human suffering and economic cost to this nation. The scope of the problem was summarized in a report entitled Back Injuries, prepared by the Department of Labor's Bureau of Labor Statistics [DOL(BLS)], Bulletin, published in 1982.
The DOL's conclusions are consistent with current workers' compensation data indicating that "injuries to the back are one of the more common and costly types of work-related injuries." According to the DOL report, back injuries accounted for nearly 20% of all injuries and illnesses in the workplace, and nearly 25% of the annual worker's compensation payments. A more recent report by the National Safety Council (Accident Facts, 1990) indicated that overexertion was the most common cause of occupational injury, accounting for 31% of all injuries. The back, moreover, was the body part most frequently injured (22% of 1.7 million injuries) and the most costly to workers' compensation systems.
More than ten years ago, the National Institute for Occupational Safety and Health (NIOSH) recognized the growing problem of work-related back injuries and published the Work Practices Guide for Manual Lifting (WPG) (DHHS(NIOSH), 1981). The WPG (1981) contained a summary of the lifting-related literature before 1981; analytical procedures and a lifting equation for calculating a recommended weight for specified two-handed, symmetrical lifting tasks; and an approach for controlling the hazards of low back injury from manual lifting. The approach to hazard control was coupled to the Action Limit (AL), a resultant term that denoted the recommended weight derived from the lifting equation.
In 1985, the National Institute for Occupational Safety and Health (NIOSH) convened an ad hoc committee of experts who reviewed the current literature on lifting, including the 1981 NIOSH WPG. The literature review was summarized in a document entitled "Scientific Support Documentation for the Revised 1991 NIOSH Lifting Equation: Technical Contract Reports, May 8, 1991," which is available from the National Technical Information Service (NTIS No. PB-91-226-274). The literature summary contains updated information on the physiological, biomechanical, psychophysical, and epidemiological aspects of manual lifting. Based on the results of the literature review, the ad hoc committee recommended criteria for defining the lifting capacity of healthy workers. The committee used the criteria to formulate a revised lifting equation (as indicated on page 4 of this application). The equation was publicly presented in 1991 by NIOSH staff at a national conference in Ann Arbor, Mich. entitled: "A National Strategy for Occupational Musculoskeletal Injury Prevention--Implementation Issues and Research Needs." Subsequently, NIOSH staff developed the documentation for the equation and played a prominent role in recommending methods for interpreting the results of the lifting equation. The revised lifting equation reflects new findings and provides methods for evaluating asymmetrical lifting tasks, and lifts of objects with less than optimal couplings between the object and the worker's hands. The revised lifting equation also provides guidelines for a larger range of work durations and lifting frequencies than the earlier equation (WPG, 1981).
The rationale and criterion for the development of the revised NIOSH lifting equation are provided in a separate journal article entitled: "Revised NIOSH Equation for the Design and Evaluation of Manual Lifting Tasks," by Waters, Putz-Anderson, Garg, and Fine, 1993. This article provides an explanation of the selection of the biomechanical, physiological, and psychophysical criterion, as well as a description of the derivation of the individual components of the revised lifting equation.
Although the revised lifting equation has not been fully validated, the recommended weight limits derived from the revised equation are consistent with, or lower than, those generally reported in the literature, and are more likely to protect healthy workers for a wider variety of lifting tasks than methods that rely only a single task factor or single criterion.
Definitions, restrictions/limitations and data requirements for the revised lifting equation are provided below.
The RWL is the principal product of the revised NIOSH lifting equation. The RWL is defined for a specific set of task conditions as the weight of the load that nearly all healthy workers could perform over a substantial period of time (e.g., up to 8 hours) without an increased risk of developing lifting-related LBP. EQU RWL=LC.times.HM.times.VM.times.DM.times.AM.times.FM.times.CM
NIOSH also developed a secondary term, the Lifting Index providing a relative estimate of the level of physical stress associated with a particular manual lifting task. The estimate of the level of physical stress is defined by the relationship of the weight of the load lifted (L) and the recommended weight (RWL). The LI is defined by the following equation: ##EQU1##
The lifting equation is a tool for assessing the physical stress of two-handed manual lifting tasks. As with any tool, its application is limited to those conditions for which it was designed. Specifically, the lifting equation was designed to meet specific lifting-related criteria that encompass biomechanical, work physiology, and psychophysical assumptions and data, identified above. To the extent that a given lifting task accurately reflects these underlying conditions and criteria, this lifting equation may be appropriately applied.
The 1991 lifting equation does not include task factors to account for unpredicted conditions, such as unexpectedly heavy loads, slips, or falls. Additional biomechanical analyses may be required to assess the physical stress on joints that occur from traumatic incidents. Moreover, if the environment is unfavorable (e.g., temperatures or humidity significantly outside the range of 19.degree. to 26.degree. C. [66.degree. to 79.degree. F.] or 35% to 50%, respectively) independent metabolic assessments would be needed to gauge the effects of these variables on heart rate and energy consumption.
The 1991 lifting equation is limited in that it was not designed to assess tasks involving one-handed lifting, lifting while seated or kneeling, or lifting in a constrained or restricted work space. The equation also does not apply to lifting unstable loads, lifting of wheel barrows, shoveling, or high-speed lifting. For such task conditions, independent and task specific biomechanical, metabolic, and psychophysical assessments may be needed.
The use of the 1991 lifting equation requires the assumption that the worker/floor surface coupling provides at least a 0.4 (preferably 0.5) coefficient of static friction between the shoe sole and the working surface. An adequate worker/floor surface coupling is necessary when lifting to provide a firm footing and to control accidents and injuries resulting from foot slippage. A 0.4 to 0.5 coefficient of static friction is comparable to the friction found between a smooth, dry floor and the sole of a clean, dry leather work shoe (nonslip type). Independent biomechanical modeling may be used to account for variations in the coefficient of friction.
The use of the 1991 lifting equation requires the additional assumption that lifting and lowering tasks have the same level of risk for low back injuries (i.e. that lifting a box from the floor to a table is as hazardous as lowering the same box from a table to the floor). This assumption may not be true if the worker actually drops the box rather than lowering it all the way to the destination. Independent metabolic and/or psychophysical assessments may be needed to assess worker capacity for various lowering conditions.
The following list of brief definitions are useful in applying the revised NIOSH lifting equation. For detailed descriptions of these terms, refer to the individual sections where each is discussed. Exemplary methods for measuring these variables and examples may be found in "Applications Manual for the Revised NIOSH Lifting Equation", by Thomas R. Waters et al, May 20, 1993.
Lifting task is defined as the act of manually grasping an object of definable size and mass with two hands, and vertically moving the object without mechanical assistance.
Load Weight (L) is the weight of the object to be lifted, in pounds, including the container.
Horizontal location (H) is the distance of the hands away from the mid-point between the ankles, in inches (measure at the origin and destination of lift). See FIG. 6.
Vertical location (V) is the distance of the hands above the floor, in inches (measure at the origin and destination of lift). See FIG. 6.
Vertical travel distance (D) is the absolute value of the difference between the vertical heights at the destination and origin of the lift, in inches.
Angle of asymmetry (A) is the angular measure of how far the object is displaced from the front (mid-sagittal plane) of the worker's body at the beginning or ending of the lift, in degrees (measure at the origin and destination of lift). See FIG. 7.
Frequency of lifting (F) is the average number of lifts per minute over a 15 minute period.
Duration of lifting is the three-tiered classification of lifting duration specified by the distribution of work-time and recovery-time (work pattern). Duration is classified as either 1, 2, or 8 hours, depending on the work pattern.
Coupling classification is the classification of the quality of the hand-to-container coupling (e.g., handle., cut-out, or grip). Coupling quality is classified as good, fair, or poor.
Significant Control is defined as a condition requiring "precision placement" of the load at a destination of the lift. This is usually the case when (1) the worker has to re-grasp the load near the destination of the lift, (2) the worker has to momentarily hold the object at the destination, or (3) the worker has to position or guide the load at the destination.
The revised lifting equation for calculating the Recommended Weight Limit (RWL), as previously set out on page 4) is based on a multiplicative model that provides a weighting for each of six task variables. The weightings are expressed as coefficients that serve to decrease the load constant, which represents the maximum recommended load weight to be lifted under ideal conditions. EQU RWL=LC.times.HM.times.VM.times.DM.times.AM.times.FM.times.CM
Where:
LC=Load Constant=51 lb PA1 HM=Horizontal Multiplier=(10/H) PA1 VM=Vertical Multiplier=1-(0.0075.vertline.V-30.vertline.) PA1 DM=Distance Multiplier=0.82+(1.8/D) PA1 AM=Asymmetric Multiplier=1-(0.0032 A) PA1 FM=Frequency Multiplier=From Table 1 PA1 CM=Coupling Multiplier=From Table 2 PA1 1. The origin and destination of the lift are oriented at an angle to each other. PA1 2. There is inadequate room to use a step turn. PA1 3. The lifting motion is across the body, such as occurs in swinging bags or boxes from one location to another. PA1 4. The lifting is done to maintain body balance in obstructed workplaces, on rough terrain, or on littered floors. PA1 5. Productivity standards require reduced time per lift. PA1 (1) The individual multipliers can be used to identify specific job-related problems. The relative magnitude of each multiplier indicates the relative contribution of each task factor (e.g., horizontal, vertical, frequency, etc.) PA1 (2) The RWL can be used to guide the redesign of existing manual lifting jobs or to design new manual lifting jobs. For example, if the task variables are fixed, then the maximum weight of the load could be selected so as not to exceed the RWL; if the weight is fixed, then the task variables could be optimized so as not to exceed the RWL. PA1 (3) The LI can be used to estimate the relative magnitude of physical stress for a task or job. The greater the LI, the smaller the fraction of workers capable of safely sustaining the level of activity. Thus, two or more job designs could be compared. PA1 (4) The LI can be used to prioritize ergonomic redesign. For example, a series of suspected hazardous jobs could be rank ordered according to the LI and a control strategy could be developed according to the rank ordering (i.e., jobs with lifting indices above 1.0 or higher would benefit the most from redesign).
The term "task variables" refers to the measurable task descriptors (i.e., H, V, D, A, F, and C); whereas, the term "multipliers" refers to the reduction coefficients in the equation (i.e., HM, VM, DM, AM, FM, and CM).
Each multiplier should be computed from the appropriate formula, but in some cases it will be necessary to use linear interpolation to determine the value of a multiplier, especially when the value of a variable is not directly available from a table. For example, when the measured frequency is not a whole number, the appropriate multiplier must be interpolated between the frequency values in the table for the two values that are closest to the actual frequency. Following is a brief discussion of the task variables, the restrictions, and the associated multiplier for each component of the model.
Horizontal location (H) is measured from the midpoint of the line joining the inner ankle bones to a point projected on the floor directly below the mid-point of the hand grasps (i.e., load center), as defined by the large middle knuckle of the hand (FIG. 6). If significant control is required at the destination (i.e., precision placement), then H should be measured at both the origin and destination of the lift.
In those situations where the H value can not be measured, then H may be approximated from the following equations: EQU H=8+W/2 for V.gtoreq.10 inches EQU H=10+W/2 for V&lt;10 inches
Where: W is the width of the container in the sagittal plane and V is the vertical location of the hands from the floor.
If the horizontal distance is less than 10 inches, then H is set to 10 inches. Although objects can be carried or held closer than 10 inches from the ankles, most objects that are closer than this cannot be lifted without encountering interference from the abdomen or hyperextending the shoulders. While 25 inches was chosen as the maximum value for H, it is probably too large for shorter workers, particularly when lifting asymmetrically. Furthermore, objects at a distance of more than 25 inches from the ankles normally cannot be lifted vertically without some loss of balance.
The Horizontal Multiplier (HM) is 10/H, for H measured in inches, and HM is 25/H, for H measured in centimeters. If H is less than or equal to 10 inches, the multiplier is 1.0. HM decreases with an increase in H value. The multiplier for H is reduced to 0.4 when H is 25 inches. If H is greater than 25 inches, then HM=0.
Vertical location (V) is defined as the vertical height of the hands above the floor. V is measured vertically from the floor to the mid-point between the hand grasps, as defined by the large middle knuckle. The coordinate system is illustrated in FIG. 6.
The vertical location (V) is limited by the floor surface and the upper limit of vertical reach for lifting (i.e., 70 inches). The vertical location should be measured at the origin and the destination of the lift to determine the travel distance (D).
To determine the Vertical Multiplier (VM), the absolute value or deviation of V from an optimum height of 30 inches is calculated. A height of 30 inches above floor level is considered "knuckle height" for a worker of average height (66 inches). The Vertical Multiplier (VM) is (1-(0.0075.vertline.V-30.vertline.)) for V measured in inches.
When V is at 30 inches, the vertical multiplier (VM) is 1.0. The value of VM decreases linearly with an increase or decrease in height from this position. At floor level, VM is 0.78, and at 70 inches height VM is 0.7. If V is greater than 70 inches, then VM=0.
The Distance variable (D) is defined as the vertical travel distance of the hands between the origin and destination of the lift. For lifting, D can be computed by subtracting the vertical location (V) at the origin of the lift from the corresponding V at the destination of the lift (i.e., D is equal to V at the destination minus V at the origin). For a lowering task, D is equal to V at the origin minus V at the destination.
The Distance variable (D) is assumed to be at least 10 inches, and no greater than (70-V) inches. If the vertical travel distance is less than 10 inches, then D should be set to the minimum distance of 10 inches.
The Distance Multiplier (DM) is (0.82+(1.8D)) for D measured in inches. For D less than 10 inches D is assumed to be 10 inches, and DM is 1.0. The Distance Multiplier, therefore, decreases gradually with an increase in travel distance. The DM is 1.0 when D is set at 10 inches; DM is 0.85 when D=70 inches. Thus, DM ranges from 1.0 to 0.85 as the D varies from 0 inches to 70 inches.
Regarding the asymmetry component, asymmetry refers to a lift that begins or ends outside the sagittal plane. In general, asymmetric lifting should be avoided. If asymmetric lifting cannot be avoided, however, the recommended weight limits are significantly less than those limits used for symmetrical lifting.
An asymmetric lift may be required under the following task or workplace conditions:
The asymmetric angle (A), which is depicted graphically in FIG. 6, is operationally defined as the angle between the asymmetry line and the sagittal line. The asymmetry line is defined as the line that joins the mid-point between the inner ankle bones and the point projected on the floor directly below the mid-point of the hand grasps, as defined by the large middle knuckle. The sagittal line is defined as the line passing through the mid-point between the inner ankle bones and lying in the sagittal plane, as defined by the neutral body position (i.e., hands directly in front of the body, with no twisting at the legs, torso, or shoulders).
The asymmetry angle (A) must always be measured at the origin of the lift. If significant control is required at the destination, however, then angle A should be measured at both the origin and the destination of the lift. The angle A is limited to the range from 0.degree. to 135.degree.. If A&gt;135.degree., then AM is set equal to zero, which results in a RWL of zero, or no load.
The Asymmetric Multiplier (AM) is 1-(0.0032A). The AM has a maximum value of 1.0 when the load is lifted directly in front of the body. The AM decreases linearly as the angle of asymmetry (A) increases. The range is from a value of 0.57 at 135.degree. of asymmetry to a value of 1.0 at 0.degree. of asymmetry (i.e., symmetric lift). If A is greater than 135.degree., then AM=0, and the load is zero.
The frequency multiplier is defined by (a) the number of lifts per minute (frequency), (b) the amount of time engaged in the lifting activity (duration), and (c) the vertical height of the lift from the floor. Lifting frequency (F) refers to the average number of lifts made per minute, as measured over a 15-minute period. Because of the potential variation in work patterns, analysts may have difficulty obtaining an accurate or representative 15-minute work sample for computing the lifting frequency (F). If significant variation exists in the frequency of lifting over the course of the day, analysts should employ standard work sampling techniques to obtain a representative work sample for determining the number of lifts per minute. For those jobs where the frequency varies from session to session, each session should be analyzed separately. In any event, the overall work pattern must still be considered. For more information, most standard industrial engineering or ergonomics texts provide guidance for establishing a representative job sampling strategy (e.g., Eastman Kodak Company, 1986).
For tasks with lifting frequencies below 0.2 lifts per minute (1 lift every five minutes), the lifting frequency is set equal to 0.2 lifts per minute.
Lifting duration is classified into three categories based on the pattern of continuous work-time and recovery-time (i.e., light work) periods. A continuous work-time period is defined as a period of uninterrupted work. Recover-time is defined as the duration of light work activity following a period of continuous lifting. Examples of light work include activities such as sitting at a desk or table, monitoring operations, light assembly work, etc. The three categories are short-duration, moderate-duration and long-duration.
Lifting frequency (F) for repetitive lifting may range from 0.2 lifts/min to a maximum frequency that is dependent on the vertical location of the object (V) and the duration of lifting see Table 1 below.
TABLE 1 ______________________________________ FREQUENCY MULTIPLIER TABLE DURATION F &lt;1 hour 1-2 hours 2-8 hours lifts/ V &lt; V .gtoreq. V &lt; V .gtoreq. V &lt; V .gtoreq. min 30 in 30 in 30 in 30 in 30 in 30 in ______________________________________ .ltoreq.2 1.00 1.00 .95 .95 .85 .85 5 .97 .97 .92 .92 .81 .81 1 .94 .94 .88 .88 .75 .75 2 .91 .91 .84 .84 .65 .65 3 .88 .88 .79 .79 .55 .55 4 .84 .84 .72 .72 .45 .45 5 .80 .80 .60 .60 .35 .35 6 .75 .75 .50 .50 .27 .27 7 .75 .70 .42 .42 .22 .22 8 .60 .60 .35 .35 .18 .18 9 .52 .52 .30 .30 .00 .15 10 .45 .45 .26 .26 .00 .13 11 .41 .41 .00 .23 .00 .00 12 .37 .37 .00 .21 .00 .00 13 .00 .34 .00 .00 .00 .00 14 .00 .31 .00 .00 .00 .00 15 .00 .28 .00 .00 .00 .00 &gt;15 .00 .00 .00 .00 .00 .00 ______________________________________
Lifting above the maximum frequency results in a RWL of 0.0. (Except for the special case of discontinuous lifting discussed above, where the maximum frequency is 15 lifts/minute.)
The FM value depends upon the average number of lifts/min (F), the vertical location (V) of the hands at the origin, and the duration of continuous lifting. For lifting tasks with a frequency less than 0.2 lifts per minute, set the frequency equal to 0.2 lifts per minute. The FM is determined from Table 1.
Regarding the coupling component, the nature of the hand-to-object coupling or gripping method can affect not only the maximum force a worker can or must exert on the object, but also the vertical location of the hands during the lift. A "good" coupling will reduce the maximum grasp forces required and increase the acceptable weight for lifting, while a "poor" coupling will generally require higher maximum grasp forces and decrease the acceptable weight for lifting.
The effectiveness of the coupling is not static, but may vary with the distance of the object from the ground, so that a good coupling could become a poor coupling during a single lift. The entire range of the lift should be considered when classifying hand-to-object couplings, with classification based on overall effectiveness. The analyst must classify the coupling as good, fair, or poor. If there is any doubt about classifying a particular coupling design, then the more stressful classification should be selected.
Based on the coupling classification and vertical location of the lift, a "Good" coupling type has a Coupling Multiplier (CM) of 1.00 (regardless of the vertical location of the object (V)); a "Fair" coupling type has a coupling multiplier of 0.95 (when V&lt;30 inches) or a coupling multiplier of 1.0 (when V.gtoreq.30 inches); and a "Poor" coupling type has a coupling multiplier of 0.90 (regardless of the vertical location of the object (V)). The CM is determined from Table 2 below.
TABLE 2 ______________________________________ COUPLING MULTIPLIER TABLE CM Coupling Type V &lt; 30 in V .gtoreq. 30 in ______________________________________ GOOD 1.00 1.00 FAIR .95 1.00 POOR .90 .90 ______________________________________
As defined earlier, the Lifting Index (LI) provides a relative estimate of the physical stress associated with a manual lifting job. ##EQU2## Where Load Weight (L)=weight of the object lifted (lbs.)
The recommended weight limit (RWL) and lifting index (LI) can be used to guide ergonomic design in several ways:
The NIOSH Recommended Weight Limit (RWL) equation and Lifting Index (LI) are based on the concept that the risk of lifting-related low back pain increases as the demands of the lifting task increase. In other words, as the magnitude of the lifting index increases, a greater percentage of the workforce is likely to be at risk for developing lifting-related low back pain. The exact shape of the risk function, however, is not known. Thus it is not possible to quantify the precise degree of risk associated with increments in the lifting index. From the NIOSH perspective, however, it is likely that lifting tasks with a LI.gtoreq.1.0 pose an increased risk for lifting-related low back pain for some fraction of the workforce (Waters et al., 1993). Therefore, based on this judgment, the lifting index may be used to identify potentially hazardous lifting jobs or to compare the relative severity of two jobs for the purpose of evaluating and redesigning them.
Waters et al, supra, contains additional examples of the application of the NIOSH equations to various lifting tasks. In each one of these examples, it is necessary to calculate multipliers to be used in the equations from measurements taken, or to take readings from existing tables to provide the necessary multipliers. This is a tedious, time consuming process requiring the layout of multiple equations to determine such parameters as vertical displacement as well as the equation multipliers (HM, VM, DM, AM) before the results of the final two equations can be calculated. This measurement process is also susceptible to user error resulting in misleading data that is often dangerous to individuals carrying out the lifting task. Consequently, there is a tendency on the part of users to ignore the use of the NIOSH equations, if a lifting task has to be done quickly, or if the task does not seem to be worth the bother of going through the extensive calculations.